Axial ion beam accelerator tube having a wobbled target

ABSTRACT

A circular target having a copper backing and a front target face material of scandiumtritide is mounted with its face perpendicular to the axis of an elongated member which is pivoted through a center hole of an end plate of a cup-shaped member wherein the center hole coincides with the longitudinal axis of an axial ion beam. A screw member having an off-center hole is adapted to threadedly engage the interior threaded walls of the cup-shaped member and through which the elongated member also passes. The rotation of the screw causes the target to revolve around the longitudinal axis of the ion beam at a varying radius as measured from the ion beam, such varying radius being a function of the rotation of the screw member. This causes the ion beam to trace a spiral pattern upon the target face. In an alternative embodiment, the elongated member is also pivoted at a median point along its length but passes through an off-center hole in a disc which is captured but free to rotate within a ring screw adapted to threadedly engage the interior walls of the cupshaped member. This embodiment provides a concentric circle pattern on the face of the target in conjunction with the ion beam.

Bergan Oct. 7, 1975 AXIAL ION BEAM ACCELERATOR TUBE HAVING A WOBBLED TARGET Primary Examiner1-Iarold A. Dixon Attorney, Agent, or FirmWilliam E. Johnson, Jr.

[ 5 7 ABSTRACT A circular target having a copper backing and a front target face material of scandiumtritide is mounted with its face perpendicular to the axis of an elongated member which is pivoted through a center hole of an end plate of a cup-shaped member wherein the center hole coincides with the longitudinal axis of an axial ion beam. A screw member having an off-center hole is adapted to threadedly engage the interior threaded walls of the cup-shaped member and through which the elongated member also passes. The rotation of the screw causes the target to revolve around the longitudinal axis of the ion beam at a varying radius as measured from the ion beam, such varying radius being a function of the rotation of the screw member. This causes the ion beam to trace a spiral pattern upon the target face. In an alternative embodiment, the elongated member is also pivoted at a median point along its length but passes through an off-center hole in a disc which is captured but free.- to rotate within a ring screw adapted to threadedly engage the interior walls of the cup-shaped member. This embodiment provides a concentric circle pattern on the face of the target in conjunction with the ion beam.

5 Claims, 5 Drawing Figures lON BEAM U.S. Patent Oct. 7,1975 Sheet 1 of3 3,911,282

US Pamnt Oct. 7,1975 Sheet 2 0f 3 3,911,282

l6 ION BEAM 26 FIG. 2

ION BEAM FIG. 3

U.S. Patent Oct. 7,1975 Sheet 3 of 3 3,911,282

FIG. 4

FIG. 5

AXIAL ION BEAM ACCELERATOR TUBE HAVING A WOBBLED TARGET BACKGROUND OF THE INVENTION.

This invention relates to artificial sources of radioactivity, and more particularly relates to methods and apparatus for extending the useful life of static atmosphere ion beam accelerators adapted to produce radioactivity by extending the useful life of the target.

It is well known that various types of radiation can be generated by suitable nuclear reactions, and artificial sources of radiation are commercially available which generate desired types of radiation by means of selected nuclear reactions. For example, 3.3 Mev neutrons may be generated with apparatus which bombards deuterium nuclei with high energy deuterons, and 14.4 Mev neutrons may be produced by bombardment of tritium with high energy deuterons. Similarly, l7 Mev gamma rays may be produced by bombardment of lithium nuclei With high energy protons.

All such artificial sources of radiation are commonly referred to as ion beam accelerators since they basically involve the acceleration of the bombarding ions (deuterons, tritons, protons, etc.) against a target to achieve the nuclear reaction sought to be obtained. Such apparatus essentially consists of a source of ions, a target, and an accelerating means for accelerating the ions into the target to produce the sought-for-radiation. In a typical D-T reactor, the source of ions includes a pair of electrodes immersedin an atmosphere of deuterium and a power supply for passing a flow of electrons between the electrodes. The target often consists of a metal body which is impregnated with tritium, and the accelerating means is a voltage supply for establishing a very high potential between the electrodes and the target. Deuterium ions created by the electrons are thus attracted or accelerated into the tritiumimpregnated target, and a 14.4 Mev neutron is generated from each interaction between a deuteron and a triton.

It is also known in the art, howeverdthat after a short period of irradiation of the target, deuterons are absorbed by the target material. The reaction of deuterium on deuterium produces He neutrons having kinetic energies of approximatelyS Mev. At the same time the current density required for the D-T reaction causes heating of the tritium target and loss of tritium gas therefrom. Thus the initial high energy source is gradually converted to a source of relatively low energy neutrons .of weak intensity and the resultant reaction with the earth formation is the same as though a spectrum of neutron energies, extending into the low energy range, were being generated for the weak intensity within the accelerator.

Because of the resulting loss of tritium, it has also been well known in the art, for example, in my own US. Pat. No. 3,400,290, to provide a movable target to thus provide what is essentially a new target for the ion beam. Other examples of analogous prior art include US. Pat. No. 2,993,996 to W. E. Meyerhof and US. Pat. No. 2,926,270 to M. J. Zunick. In my US. Pat. No. 3,400,290, the ion beam is not an axial beam, the ion beam embodied within my patent is a lateral beam directed at a sleeve target perpendicular to the longitudinal axis of the accelerator tube and the sleeve target is arranged to be moved in a direction perpendicular to the ion beam and coaxially with the longitudinal axis of the accelerator tube. The Meyerhof US. Pat. No. 2,993,996 teaches the use of a moving target pulled along as a tape in one embodiment and the use of a rotating target having a series of gears and motors for providing a spiral impingement of the ion beam on the rotating target. It should be appreciated, however, that the Meyerhof embodiments are extremely complicated in depending upon such a motorized gear arrangement.

The Zunick US. Pat. No. 2,926,270 relates to a rotating anode used in an X-ray tube wherein a wobbletype device is described such the angle of wobble about a pivot point is controlled by .a solenoid attached to the end of the elongated member away from the target end. As with the Meyerhof device, the Zunick device is rather complicated and involves use of various means to achieve rotation and to cause the elongation member to pivot which are impractical in a well logging apparatus.

It is therefore the primary object of the present invention to provide a new and improved method and apparatus for extending the life of a target in a borehole accelerator useful in well logging;

It is also an object of the invention to provide methods and apparatus for improving the target life in borehole accelerators which are vastly more simple than the complicated arrangements known in the prior art.

The objects of the invention are accomplished, generally, by the provision of methods and apparatus which incorporate an elongated member having first and second ends and upon a first end of which the target is mounted, means to pivot the elongated member at an intermediate point along its length, and means to rotate the face of the target about the longitudinal axis of a beam of ions, wherein the target rotates about a radius as measured from the ion beam which varies in a manner functionally related to the rotation of the target. In an alternative embodiment, the rotation means and radius-varying means are independently controlled.

These and other objects, features and advantages of the present invention will be more readily appreciated from a reading of the following detailed specification and drawing, in which:

FIG. 1 is a schematic illustration, partly in cross section, of the interior of a borehole accelerator according to the present invention;

FIG. 2 is a cross-sectional illustration of the subassembly housing apparatus which is used to vary the location of the target exposed. to the ion beam in accordance with the present invention;

FIG. 3 is a cross-sectional illustration of an altema' tive embodiment according to the invention;

FIG. 4 is a plan view of the target pattern in accordance with the apparatus according to FIG. 2; and

FIG. 5 is a plan view of the target pattern in accordance with the apparatus according to FIG. 3.

Referring now to the drawing in more detail, especially to FIG. 1, there is schematically illustrated the interior of a portion of a borehole accelerator which finds as its primary utility a source for radioactivity well logging. It should be appreciated that borehole accelerators are known per se, for example, my US. Pat. No. 3,400,290. The housing 10 includes within its interior an ion source .11 and a high voltage supply 12 separated by high voltage insulator 18. Such ion sources are conventional and no further description is needed for purposes of the present invention. Suffice it to say at this point that the ion source 11 produces an axial beam of ions 13 which may or may not be perfectly focused depending upon the space available within the housing 10. Also within the housing there is located a subassembly housing 14 (shown in greater detail in FIGS. 2 and 3) having a target 15 attached .with its face perpendicular to the axis of an elongated member 16 and which has a sealing bellows 17 to maintain a sealed atmosphere, commonly known as a static atmosphere, within the interior of the housing 10 around the ion source 11 and target 15.

Referring now to FIG. 2, the interior of the subassembly housing 14 is illustrated in greater detail. The housing 14 includes a cup-shaped member having a front plate 20 which is securely attached to the exterior wall 21 of the housing 10, for example, as by welding, and has the necessary seals to maintain the static atmosphere within the interior of the housing 10 containing the target 15. The front plate 20 includes a pivot hole 22 through which the elongated member 16 extends. The back side of the front plate 20 includes a much larger hole 23 to allow the higher degree of pivoting of the elongated member 16. The small pivot hole .22 is preferably axially aligned with the ion beam 13. The cup-shaped member also includes cup walls 24 having interior threads.

A screw 25 having external threads to match the internal threads of the cup walls 24 has a slot 26 for enabling the screw 25 to be advanced or withdrawn within the confines of the cup walls 24. The screw 25 includes an off-center hole, i.e., off-center with respect to the ion beam 13, the off-center hole preferably including a pair of holes 27 and 28 through which the elongated member 16 extends.

The target 15 preferably includes a copper substrate with a scandium-tritidedisc on its front side facing the ion beam 13. v

in the operation of the apparatus according to H6. 2, it should be appreciated that since the pivot point within the hole 22 is axially aligned with the ion beam 13 and the hole comprised of the two holes 27 and 28 within the screw 25 is off center, the rotation of the screw 25 causes the target 15 to revolve around the ion beam with a varying radius. which describes a spiral as is illustrated in FIG. 4. As the screw 25 is advanced into the cups, the spiral approaches the outside edge of the face of the target 15 and as the screw is withdrawn, the ion beam approaches the center of the spiral on the targ Referring now to FIG. 3, there is illustrated an alternative embodimentof the invention wherein all of the parts are similar to those illustrated in FIG. 2 except for that portion of the sub-assembly housing 14 which is threaded into the internal threads of the cup walls 24. A ring screw 30 has a slot 31 for advancing the screw ring 30. A disc 32 is captured within a slot 33 inside the screw ring 30 such that the disc 32 is free to rotate within the slot 33. The disc 32 has a holewhich is off center from the longitudinal axis of the ion l3 and the pivot hole 22. The off-center hole 34 has the elongated member 16 extending therethrough. The disc 32 also has a slot 35 for rotating the disc 32 within the slot 33.

In the operation of the apparatus according toFlG. 3, the ring screw 30 can'be rotated to move the ring screw in or out of the cup walls 24. This causes the target 15' to be pivoted up or down with regard tothe pivot point defined by the hole 22. For each increment that the ring screw advances inward, the disc 32 may be advanced 360, thus causing the ion beam to describe concentric circles of increasing diameter until the target is uniformly depleted of tritium down to the minimum acceptable neutron yield. The concentric circle pattern is illustrated in FIG. 5.

Thus there have been illustrated and described herein the preferred embodiments of the present invention wherein means are provided for improving the life of a target in a borehole accelerator. Those skilled in the art, however, will recognize that obvious modifications to these embodiments'can be made without departing from the scope of the invention. For example, other types of target materials may be employed, as well as means other than bellows for sealing the interior to provide a static atmosphere around the target. Also, other means may be provided for rotating the screw to be advanced or withdrawn, for example, as by the turning of anextemal-bolt attached thereto. It should also be appreciated that the invention contemplates the use of a pivot point which does not coincide with the longitudinal axis of the ion beam but which nonetheless allows at least a portion of the target face to rotate about the ion beam.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. in a static atmosphere ion beam accelerator having a beam source of ions, means for accelerating said ions and a target for said accelerated ions, the improvement comprising: 1

an elongated member having first and second ends and upon a first end of which said target is mounted;

a housing within said accelerator providing a pivot for said elongated member between said first and second ends; and

means between said second end and said pivot for rotating the target mounted on said first end of said elongated member about said pivot and about the longitudinal axis of said beam of ions, thereby causing the accelerated ions to trace a path of varying radius on said target in a manner functionally related to said rotation.

2.'The static atmosphere ion beam accelerator according to claim 1 wherein said means to pivot and said means to rotate comprise a cup-shaped member having an end plate with a pivot hole through which said elongated member extends and a screw adaptedto be threadedly advanced through the walls of said cupsh'aped member and having a hole through which said elongated member extends and which is offsetwith respect to the longitudinal axis of said ion beam.

3. The static atmosphere ion beam accelerator according to claim 2 wherein said pivot hole coincides with the longitudinal axis of said ion beam.

4. In a static atmosphere ion beam accelerator having a beam source of v ions, means for accelerating said ions and a target for said accelerated ions, the improvement comprising:

an elongated member having first and second ends and upon a first end of which said target is mounted; means to pivot said elongated member at an intermediate point along its length;

means to rotate the targe mounted on said first end which is offset with respect to the longitudinal axis of said elongated member about the longitudinal of said ion beam and further comprising a housing axis of said beam of ions; and adapted to threadedly receive said screw ring means to vary the radius of said rotation of said target 5. The static atmosphere ion beam accelerator acas measured from said ion beam, said means to 5 cording to claim 4 wherein said means to rotate said vary the radius comprising a screw ring having a target comprises said disc being free to rotate within a disc captured within said screw ring having a hole groove within said screw ring. through which said elongated member extends and 

1. In a static atmosphere ion beam accelerator having a beam source of ions, means for accelerating said ions and a target for said accelerated ions, the improvement comprising: an elongated member having first and second ends and upon a first end of which said target is mounted; a housing within said accelerator providing a pivot for said elongated member between said first and second ends; and means between said second end and said pivot for rotating the target mounted on said first end of said elongated member about said pivot and about the longitudinal axis of said beam of ions, thereby causing the accelerated ions to trace a path of varying radius on said target in a manner functionally related to said rotation.
 2. The static atmosphere ion beam accelerator according to claim 1 wherein said means to pivot and said means to rotate comprise a cup-shaped member having an end plate with a pivot hole through which said elongated member extends and a screw adapted to be threadedly advanced through the walls of said cup-shaped member and having a hole through which said elongated member extends and which is offset with respect to the longitudinal axis of said ion beam.
 3. The static atmosphere ion beam accelerator according to claim 2 wherein said pivot hole coincides with the longitudinal axis of said ion beam.
 4. In a static atmosphere ion beam accelerator having a beam source of ions, means for accelerating said ions and a target for said accelerated ions, the improvement comprising: an elongated member having first and second ends and upon a first end of which said target is mounted; means to pivot said elongated member at an intermediate point along its length; means to rotate the targe mounted on said first end of said elongated member about the longitudinal axis of said beam of ions; and means to vary the radius of said rotation of said target as measured from said ion beam, said means to vary the radius comprising a screw ring having a disc captured within said screw ring having a hole through which said elongated member extends and which is offset with respect to the longitudinal axis of said ion beam and further comprising a housing adapted to threadedly receive said screw ring.
 5. The static atmosphere ion beam accelerator according to claim 4 wherein said means to rotate said target comprises said disc Being free to rotate within a groove within said screw ring. 